Ablation system, methods, and controllers

ABSTRACT

Multi-electrode ablation systems, methods, and controllers are described. In one example, a method of beginning an ablation procedure using a multi-electrode ablation system is described. The method includes selectively coupling the output of a power supply to a first electrode of a plurality of electrodes to increase a temperature at the first electrode to a first temperature set-point and limit a rate of increase of the temperature at the first electrode to a predetermined first rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application Ser. No.61/788,012, filed Mar. 15, 2013, and to provisional application Ser. No.61/817,550, filed Apr. 30, 2013, and to provisional application Ser. No.61/817,561, filed Apr. 30, 2013, each of which is incorporated herein inits entirety.

BACKGROUND OF THE DISCLOSURE a. Field of the Disclosure

The present disclosure relates generally to ablation systems, methods,and controllers. More particularly, the present disclosure relates tomulti-electrode ablation systems, methods, and controllers.

b. Background Art

It is known that ablation systems are used to perform ablationprocedures to treat certain conditions of a patient. A patientexperiencing arrhythmia, for example, may benefit from ablation toprevent irregular heart beats caused by arrhythmogenic electric signalsgenerated in cardiac tissues. By ablating or altering cardiac tissuesthat generate such unintended electrical signals, the irregular heartbeats may be stopped. Ablation systems are also known for use intreating hypertension in patients. In particular, renal ablationsystems, also referred to as renal denervation systems, are used tocreate lesions along the renal sympathetic nerves—a network of nervesthat help control blood pressure. The intentional disruption of thenerve supply has been found to cause blood pressure to decrease.

Known techniques for renal denervation typically connect a radiofrequency (“RF”) generator to a catheter. The catheter is inserted inthe renal artery and RF energy is emitted through an electrode in thedistal end of the catheter to heat the renal nerves to a temperaturethat reduces the activity of renal nerve(s) near the electrode. Theelectrode is repositioned to several locations around the innercircumference and the length of the artery during the process. Somerenal denervation systems utilize a catheter with more than oneelectrode in order to reduce the number of times that the catheter mustbe repositioned during the denervation procedure. Some of these systemsapply RF energy to the multiple electrodes sequentially, while othersapply the RF energy to all of the electrodes simultaneously. In somesystems that separately control the RF energy delivered to multipleelectrodes, multiple power supplies are used to provide the RF energy tothe electrodes.

Moreover, ablation is achieved by applying heat to the selected area(s)over time. Thus, it is important to regulate the amount of heat, andmore particularly the temperature at each electrode, during treatment ofthe patient. Mechanical differences between electrodes, dissimilarcontact qualities between the electrodes and the treatment area (e.g.,the artery wall when using a renal denervation system), and otherfactors result in different power levels being required for each of themultiple electrodes to achieve a desired temperature setpoint.

There is a need, therefore, for multi-electrode ablation systems thatoperate multiple electrodes simultaneously, efficiently, and accuratelyto regulate the temperature at the electrodes.

BRIEF SUMMARY OF THE DISCLOSURE

In one aspect, a method of beginning an ablation procedure using amulti-electrode ablation system includes selectively coupling the outputof a power supply to a first electrode of a plurality of electrodes toincrease a temperature at the first electrode to a first temperatureset-point and limit a rate of increase of the temperature at the firstelectrode to a predetermined first rate.

In another aspect, a multi-electrode ablation system includes a powersupply configured to be coupled to a plurality of electrodes, and acontroller coupled to the power supply. The controller is configured toselectively couple the output of the power supply to a first electrodeof the plurality of electrodes to increase a temperature at the firstelectrode to a first temperature set-point and limit an increase of thetemperature at the first electrode to a predetermined first rate.

In still another aspect, a method of beginning an ablation procedureusing a multi-electrode ablation system includes increasing atemperature at each electrode of a plurality of electrodes until thetemperature at each electrode reaches a first temperature set-point. Arate of increase of the temperature at each electrode is limited to apredetermined first rate of increase.

Another aspect is a multi-electrode ablation system including a powersupply configured to be coupled to a plurality of electrodes and acontroller coupled to the power supply. The controller is configured toincrease a temperature at each electrode of the plurality of electrodesuntil the temperature at each electrode reaches a first temperatureset-point. The rate of increase of the temperature at each electrode islimited to a predetermined first rate of increase.

The foregoing and other aspects, features, details, utilities andadvantages of the present disclosure will be apparent from reading thefollowing description and claims, and from reviewing the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of one embodiment of an ablation systemincluding a generator, a catheter, and a return electrode.

FIG. 2 is a partial view of a distal end of the catheter shown in FIG.1.

FIG. 3 is a schematic block diagram of a controller for use in thegenerator shown in FIG. 1.

FIG. 4 is a functional block diagram of the ablation system shown inFIG. 1.

FIG. 5 is a digital signal processor (DSP) for use in the ablationsystem shown in FIG. 1.

FIG. 6 is a diagram of the operating states of the system shown in FIG.1.

FIG. 7 is a diagram of a control cycle for use with the system shown inFIG. 1.

FIG. 8 is an equivalent circuit diagram of the system shown in FIG. 1when in use within an artery.

FIG. 9 is a diagram of an output cycle of the system shown in FIG. 1with four electrodes enabled.

FIG. 10 is a diagram of an output cycle of the system shown in FIG. 1with three electrodes enabled.

FIG. 11 is a graphical representation of an output cycle of the systemshown in FIG. 1.

FIG. 12 is a timing diagram of electrode switching signals in the systemshown in FIG. 1.

FIG. 13 is a graphical representation of an output signal waveform fromthe generator shown in FIG. 1.

FIG. 14 is a graphical presentation of samples taken from the outputsignal waveform shown in FIG. 13 graphed as a function of the outputsignal phase.

FIG. 15A is a graph of a temperature set-point and an electrodetemperature during a computer simulated ablation.

FIG. 15B is a graph of the power applied to the electrode during thesimulated ablation shown in FIG. 15A.

FIG. 15C is a graph of the thermal gain of the electrode during thesimulated ablation shown in FIG. 15A

FIG. 16A is a graph of a temperature set-point and an electrodetemperature during a computer simulated ablation.

FIG. 16B is a graph of the power applied to the electrode and a powerlimit during the simulated ablation shown in FIG. 16A.

FIG. 16C is a graph of the actual thermal gain of the electrode and thecalculated thermal gain of the electrode during the simulated ablationshown in FIG. 16A.

FIG. 17 is a graph of electrode temperature, temperature set-point, andpower delivered to the electrode during a bench simulated ablation.

FIG. 18 is a graph of electrode temperature, thermal gain, power limit,power delivered to the electrode, and energy dissipated through theelectrode during an animal ablation test.

FIG. 19 is a graph of electrode temperature, thermal gain, power limit,power delivered to the electrode, and energy dissipated through theelectrode during another animal ablation test.

FIG. 20 is a graph of electrode temperature, thermal gain, power limit,power delivered to the electrode, and energy dissipated through theelectrode during an animal ablation test.

FIG. 21 is graph of an electrode temperature during a simulatedablation.

FIG. 22 is graph of electrode temperature for four electrodes during asimulated ablation.

FIG. 23 is graph of electrode temperature with a two stage ramp during asimulated ablation.

FIG. 24 is a flow diagram of an implementation of adaptive powerlimiting in an ablation system.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE DRAWINGS

This disclosure relates generally to ablation systems, methods, andcontrollers. More particularly, this disclosure relates tomulti-electrode ablation systems, methods, and controllers. Still moreparticularly, this disclosure relates to multi-electrode renal ablationsystems, methods, and controllers.

The methods and systems described herein provide accurate and efficientcontrol of a multi-electrode ablation system. In general, various noveltechniques for separately controlling when, how long, and how muchenergy to dissipate through each electrode in a multi-electrode systemare described. For example, one exemplary system providestime-multiplexed simultaneous delivery of ablation power to multipleablation electrodes with a single power supply. The energy required tomeet the energy demand of each electrode is calculated and a duty cyclefor each electrode is set accordingly. The output voltage of the singlepower supply is selected to provide sufficient power for the electrodewith the highest energy demand. A common return path resistance isdetermined using a single current sensor in the return path of amulti-electrode ablation system and used to control one or more aspectsof operation of the system.

Referring now to the drawings and in particular to FIGS. 1-4, anablation system, generally indicated at 100, includes an ablationgenerator 102, a multi-electrode ablation catheter 104, and a returnelectrode 106. The ablation catheter 104 is removeably coupled to theablation generator 102 by a cable 108. The return electrode 106 isremoveably coupled to the ablation generator 102 by a cable 110. In use,the return electrode 106 is placed externally against a patient's bodyand the catheter 104 is inserted into the patient's body. Generally, theablation generator 102 outputs radio frequency (RF) energy to thecatheter 104 through the cable 108. The RF energy leaves the catheter104 through a plurality of electrodes 112 (shown in FIG. 3) located atthe distal end 114 of catheter 104. The RF energy travels through thepatient's body to the return electrode 106. The dissipation of the RFenergy in the body increases the temperature near the electrodes,thereby permitting ablation to occur. In the exemplary embodiment setforth herein, the ablation system 100 is a renal ablation systemsuitable for use in performing renal denervation. It is understood,however, that the ablation system may be used for other treatmentswithout departing from the scope of this disclosure.

The generator 102 includes a user interface (UI) portion 116 fordisplaying information and notifications to an operator and receivinginput from the user. Display devices 118 visually display information,such as measured temperatures, power output of the generator,temperature thresholds, cycle time, etc., and/or notifications to theuser. Display devices 118 may include a vacuum fluorescent display(VFD), one or more light-emitting diodes (LEDs), liquid crystal displays(LCDs), cathode ray tubes (CRT), plasma displays, and/or any suitablevisual output device capable of displaying graphical data and/or text toa user. The indicators 120 provide visual notifications and alerts tothe user. In other embodiments, one or more of the indicators 120provide audible notifications and/or alerts to the user. In theillustrated embodiment, indicators 120 are lights, such as lightemitting diodes, incandescent lamps, etc. The indicators 120 may beturned on or off, for example, to indicate whether or not the generator102 is receiving power, whether or not the catheter 104 is connected,whether or not the catheter (or all electrodes 112) are functioningproperly, etc. Moreover, the indicators 120 may indicate a quality ordegree of a feature or component of the system 100, such as by changingcolor, changing intensity, and/or changing the number of the indicators120 that are turned on. Thus, for example, an indicator 120 may changecolor to represent a unitless notification of the quality of the contactbetween one or more of the electrodes 112 and an artery wall. UI portion116 includes inputs 122, e.g., buttons, keys, knobs, etc., for receivingcommands and/or requests from a user. In some embodiments, the UIportion 116, additionally or alternatively, displays a graphical userinterface to a user, such as via one or more of the display device 118.

As shown in FIG. 2, the multiple electrodes 112 may be disposed on abasket 124 located at the distal end 114 of the catheter 104. In theillustrated embodiment, basket 124 is an expandable basket that may beexpanded and collapsed by an operator of the system 100 to positionelectrodes 112 against, for example, an artery wall. In the illustratedembodiment, the catheter 104 includes four electrodes 112. In otherembodiments, the catheter 104 may include at least two, but other thanfour, electrodes 112. A thermocouple (not shown, also referred to hereinas a temperature sensor) is attached to each electrode 112 providestemperature readings of the electrode. The catheter 104 also contains athermistor (not shown) and a 1-Wire EEPROM. The generator 102 uses thethermistor for measuring ambient temperature and performingcold-junction compensation on the thermocouples. The EEPROM contains aunique ID which allows the generator 102 to reject devices notmanufactured specifically for use with the generator 102. The generator102 also maintains usage data on the EEPROM in order to enforce maximumoperation limits for the catheter 104.

Referring now to FIG. 3, generator 102 includes a power supply 126, acontroller 128, and an RF output circuit 130. Power supply 126 receivesAC power via an input 132 and converts the received power to a DC poweroutput. The DC power output is provided to the RF output circuit 130that outputs RF power to the catheter 104, and more specifically to theelectrodes 112, via output 134. In the exemplary embodiment, powersupply 126 includes a buck converter to provide a DC output of a lessermagnitude than the magnitude of the rectified AC power input. Thecontroller 128 is coupled to and controls operation of the power supply126 and the RF output circuit 130. As will be described in more detailbelow, the controller 128 controls operation of power supply 126 tocause the power supply to generate a desired output voltage, i.e. a DCoutput voltage having a magnitude determined by the controller 128. Inother embodiments, the power supply 126 includes its own controllerconfigured to control operation of the power supply 126 to generate thedetermined output voltage in response to a command from the controller128. The controller 128 also controls operation of the RF output circuit130. Controller 128 controls when and to which electrodes 112 the RFoutput circuit 130 couples its RF power output. In other embodiments,the RF output circuit 130 includes its own controller configured tocontrol operation of the RF output circuit 130 in response to commandsfrom the controller 128. In some embodiments, the RF output circuit 130is part of, and integrated into, the power supply 126.

The controller 128 includes a processor 136 and a memory device 138coupled to the processor 136. The term “processor” refers hereingenerally to any programmable system including systems andmicrocontrollers, reduced instruction set circuits (RISC), applicationspecific integrated circuits (ASIC), programmable logic circuits, fieldprogrammable gate array (FPGA), gate array logic (GAL), programmablearray logic (PAL), digital signal processor (DSP), and any other circuitor processor capable of executing the functions described herein. Theabove examples are exemplary only, and thus are not intended to limit inany way the definition and/or meaning of the term “processor.” Moreover,although a single processor is illustrated in FIG. 3, the processor 136may include more than one processor and the actions described herein maybe shared by more than one processor.

The memory device 138 stores program code and instructions, executableby the processor 136. When executed by the processor 136, the programcode and instructions cause the processor 136 to operate as describedherein. The memory device 138 may include, but is not limited to onlyinclude, non-volatile RAM (NVRAM), magnetic RAM (MRAM), ferroelectricRAM (FeRAM), read only memory (ROM), flash memory and/or ElectricallyErasable Programmable Read Only Memory (EEPROM). Any other suitablemagnetic, optical and/or semiconductor memory, by itself or incombination with other forms of memory, may be included in the memorydevice 138. The memory device 138 may also be, or include, a detachableor removable memory, including, but not limited to, a suitablecartridge, disk, CD ROM, DVD or USB memory. Although illustratedseparate from the processor 136, memory device 138 may be integratedwith the processor 136.

FIG. 4 is a functional block diagram of the ablation system 100. Thecontroller 128 includes a primary DSP 156, a secondary DSP 158, and FPGA160, and a user interface (UI) processor 162. The DSPs 156 and 158control the temperature and output power delivered by the four ablationelectrodes 112 contained in the catheter 104 by sending appropriatecontrol signals to the FPGA 160. The FPGA 160 controls the powerelectronics (i.e., power supply 126 and RF output circuit 130), subjectto control inputs from the UI processor 162. Both primary and secondaryDSPs 156 and 158 communicate with the FPGA 160, the UI 162, and eachother over a CAN bus 164. The primary and secondary DSPs 156 and 158also communicate certain information to each other via a dedicated McBSPlink 165.

In the illustrated embodiment, the UI processor 162 is responsible forpresenting, such as via user interface portion 116, the current state ofthe system to the operator as well as providing a means for the operatorto modify parameters such as ablation time and temperature. The UIprocessor 162 is also responsible for managing firmware upgrades,including the communication of new firmware images to the FPGA 160 andDSPs 156 and 158.

The FPGA 160 responds to control signals received from the primary DSP156 via the CAN bus 164 to drive the power supply 126 and the RF outputcircuit 130 (specifically a buck regulator and an RF amplifier,respectively, in this implementation) that apply power to the catheter104. It checks for periodic inputs from both the primary DSP 156 and thesecondary DSP 158 in order to allow an RF output, and it monitorsphysical signals from an operator foot switch (not shown) and sendscorresponding switch status updates to the DSPs 156 and 158 and to theUI processor 162.

The primary DSP 156 runs the main control loop, sampling the current,voltage, and temperature for each electrode, and adjusting the powerdelivered to each electrode to achieve the desired temperature, as willbe described in more detail below. It is also the operating state masterof the generator 102. While the operator interacts with the UI and withother physical components connected to the generator 102 in ways thatmay result in a generator state change, the primary DSP 156 ultimatelydecides the state of the generator 102.

The primary DSP 156 is supervised in its task by the secondary DSP 158,which ensures that patient safety limits are not breached. The secondaryDSP 158 independently measures output power, temperature, and otherparameters set by the primary DSP 156. The secondary DSP 158 alsoverifies that the software running on the primary DSP 156 isoperational.

FIG. 5 is a block diagram of the primary DSP 156. Secondary DSP 158 issubstantially the same as the primary DSP 156 shown in FIG. 5. In theillustrated embodiment, both the primary and secondary DSPs 156 and 158are 32-bit architecture microcontrollers each including a 32 bitprocessor 136. The DSP 156 supports a maximum clock frequency of 80 MHzand includes a floating-point unit (FPU) for native single-precisionfloating-point operations, a fully-programmable control law accelerator(CLA) 166 for offloading time-critical operations from the processor136, and a Viterbi control unit (VCU) for supporting complex mathoperations. The DSPs 156 and 158 also include embedded memory 138. TheDSPs 156 and 158 contain 4 types of memory: RAM, Flash, OTP (One-TimeProgrammable) and ROM. Flash memory is divided into 8 sectors of equalsize, for a total size of 256 Kbytes. A small amount (1K 16-bit words)of OTP memory is provided for storage of code or data that should not beerasable. This typically includes data such as serial numbers anddevice-specific addresses. 32K 16-bit words of Boot ROM are provided,which contain boot code to initialize the processor 136, varioustrigonometric data tables, such as Sine, Cosine, Arc Tangent and Taylorseries coefficients. Also available are functions to erase and programthe Flash memory.

The DSPs 156 and 158 include a set of internal peripherals 168. In orderto avoid burdening the processor 136 with simple I/O and communicationsdata transfer, a DMA controller 170 is provided to manage access to thememory 138, analog-to-digital converter (ADC) 172, USB 173, pulse widthmodulator (PWM) modules 174, and McBSP. Data transfers to and from anyof these peripherals are possible without processor 136 intervention,increasing data throughput through the system 100. The ADC 172 has twosample-and-hold circuits that can be sampled either simultaneously orsequentially. Each circuit is fed by one of 16 channels. The ADC 172 has12-bit resolution. Both the primary DSP 156 and secondary DSP 158 usethe ADC 172 to sample voltage, current, and thermocouple temperature foreach electrode 112 as well as thermistor temperature and hand switchstatus.

For generating square waves with specific frequencies and duty cycles,the DSPs 156 and 158 each contain eight PWM modules 174, each PWM module174 including two separate outputs. These modules are used forgenerating specific pulse trains. The PWM modules 174 are chainedtogether by a clock synchronization scheme that allows them to operateas a single system when required. They support deadband generation withindependent rising-edge and falling-edge delay control as well as PWMchopping by a high-frequency carrier signal. Also, several key registerscontrolling the PWM modules 174, including the time-base period andcounter-compare registers, are able to be asynchronously updated withoutcorruption or unwanted behavior through the use of shadow registers. Theprimary DSP 156 uses four PWM modules 174 to send on and off pulses tothe pulse transformers (not shown) which provide power output to thefour catheter electrodes 112. For each PWM module 174, on pulses aregenerated with one channel and off pulses with the other. In addition tobeing set for desired duty cycles, the compare values also include someoverlap between channels to minimize the amount of time that noelectrode 112 is connected to the output.

The DSPs 156 and 158 each include four high-resolution capture (HRCAP)modules 176 for taking high-resolution pulse width measurements. Eachmodule includes a dedicated input capture pin, a 2-word FIFO forrising-edge captures, and a 2-word FIFO for falling-edge captures. Thesecondary DSP 158 uses the HRCAP modules 176 to measure the duty cycleof the output at each electrode.

The CLA 166 is an on-chip floating-point co-processor that has extensiveaccess to the ADC 172 and PWM modules 174. The CLA 166 is intended to becapable of executing control loop software without intervention by themain processor 136, thereby freeing up processing bandwidth for othertasks. The CLA 166 has its own instruction set, with support foraddition, subtraction, multiplication, reciprocal, and square rootcalculation of single-precision floating point operands in four 32-bitresult registers. Conversion to and from floating point of 16 and 32-bitintegers is supported as well as the usual logical operations includingarithmetic and logical shift. Branches and loops are supported and theexistence of an indirect addressing mode in addition to the standarddirect mode facilitates the processing of structured data. Throughput isenhanced by the provision of instructions that perform concurrentoperations, such as multiply with parallel subtract and the ADC'sfeature of raising an “early interrupt” when starting its conversionprocess makes it possible for a program task to time execution so as touse the converted result “just-in-time”.

The CLA's programs consist of tasks or interrupt service routines, whichare code sequences whose starting address is contained in the interruptvector register of its associated interrupt. When this interrupt fires,typically due to an ADC 172 conversion start or completion, the task isscheduled and run. Communication with the processor 136 is effectedeither through shared data memory blocks, which are permanently readableand writable by both CLA 166 and processor 136, or by selectivelymapping memory areas for use by the CLA 166, access being controlled byaccess bits previously set by the processor 136.

Referring now to FIG. 6, which is a diagram of the operating states 600of the ablation system 100, upon system power up the software enters atemporary testing state 602 where all required subsystems are tested forcorrect operation. Examples include RAM tests and a cyclic redundancycheck (CRC) of the software executable image. If all tests completesuccessfully, the system changes to a ready state 604. While in theready state, patient information is entered and ablation parameters areconfigured on the UI 116. Meanwhile, the DSP software is also checkingfor catheter 104 presence and validity.

If one or more tests in the Check state fail, the system 100 transitionsto an error state 606. This is a non-recoverable state and can only beexited by a power cycle. Other errors detected in the system are treatedeither as nonrecoverable errors or as recoverable faults. If arecoverable fault occurs, the system transitions to a fault state 608.If a recoverable fault is cleared, the system transitions back to theready state 604.

If an update is necessary and the DSP software is in the ready state604, an update state 610 is entered, which disables all normalfunctionality and stores a new software image in flash memory. Once thenew image has been successfully transferred, a power cycle is necessaryto allow the new software to be loaded and run. If there is acommunication issue during the image transfer or if the image istransferred but ends up being invalid after a CRC check (during testingstate 602), the DSP software will request a retry from the UI 116. If avalid image is not successfully transferred after several retries, thesystem 100 will transition to the error state 606.

From the ready state 604, the system may transition to a diagnosticstate 612 once a valid catheter 104 is connected, a message signifiesthat user configuration is complete, and a valid activation switch pressis detected. During the diagnostic state 612, low-power measurements aretaken in order to perform pre-ablation electrode 112 checks. The system100 will be switched back to the ready state 604 on reception of a UI116 message indicating that the operator has requested the transition.

If a valid activation switch press is detected while in the diagnosticstate 612, the system will transition to the ablation state 614. In thisstate, all power and temperature control loops are activated, and thesecondary DSP 158 performs its supervisory functions over the primaryDSP 156. The system 100 transitions back to the ready state 604 if theconfigured ablation time is met (typically 90 seconds) or if anactivation switch is pressed again.

The primary DSP 156 is the generator state machine “master.” All statetransitions are initiated by the primary DSP 156. If another processordesires that the generator transition to another state, it must firstrequest that the primary DSP 156 make the transition. The secondary DSP158 supervises the primary DSP 156 here as well and verifies that anystate transition is valid.

The controller 128 is configured to control overall operation of thesystem 100 in concert with a user's instructions. In general, thecontroller 128 is configured, such as by instructions stored in thememory device 138, to simultaneously electrically couple the outputvoltage from the power supply 126 to electrodes 112 via the RF outputcircuit 130. Under some circumstances, such as because of a malfunctionof the electrode, operator selection, etc., one or more of theelectrodes 112 may be disabled and the disabled electrode(s) are notcoupled to the output voltage.

The primary DSP 156 is responsible for regulating the temperature ateach electrode 112, subject to oversight by the secondary DSP 158. Withadditional reference to FIGS. 7-12, the controller 128 and DSP 156 inparticular, uses an outer loop 178 (shown in FIG. 7) based ontemperature and an inner loop 180 (shown in FIG. 7) based on voltage tocontrol system 100 using a plurality of fixed length output cycles 151(shown in FIGS. 9-12). In the exemplary embodiment, each output cycle151 lasts for five milliseconds. Other embodiments may use any othersuitable length output cycle 151. As will be described in more detailbelow, each output cycle 151 includes a measurement period 150 duringwhich various measurements are taken and an output period 154 duringwhich the output voltage is coupled to one or more of the electrodes112. If more than one electrode 112 is enabled, the measurement period150 includes a plurality of measurement sub-periods, as will bedescribed in more detail below.

Mechanical differences between electrodes 112, dissimilar contactqualities between the electrodes 112 and the artery wall, and otherfactors result in different power levels being required for eachelectrode 112 to achieve the same temperature set-point, e.g., a desiredtemperature to produce ablation. Generally, the primary DSP 156 controlstemperature at each electrode 112 by modifying the output of a singlebuck regulator and exposing each of the electrodes 112 to the resultingoutput voltage for varying amounts of time (thus delivering varyingamounts of energy to each of the electrodes 112). The same outputvoltage is applied to each of the electrodes 112. The energy dissipatedthrough each electrode 112, and therefore the temperature generatedadjacent the electrode 112, is determined by how long each electrode 112is coupled to the output voltage. Because, the output cycle 151 lastsfor a fixed length of time, the maximum energy that may be delivered toany electrode 112 is determined by the output voltage of power supply126. By increasing the output voltage of power supply 126 in the innerloop 180, controller 128 increases the maximum amount of energy that maybe dissipated through an electrode (in particular an electrode 112 thatis coupled to the voltage for the entire output period). Similarly,decreasing the output voltage of the power supply 126 decreases themaximum power dissipation through the electrodes 112.

With respect to the outer loop 178 of the control system, the differencein desired temperature versus measured or actual temperature, i.e., atemperature difference, is used to determine a desired power for eachelectrode 112. Both the primary and secondary DSPs 156 and 158 (shown inFIG. 4) sample temperature values. Due to the limited number ofsample-and-hold circuits in the ADC 172 (shown in FIG. 5), the DSPs 156and 158 collect these measurements at times other than when the voltageand current sampling is taking place. Therefore, all temperaturemeasurements occur in the output period 154 of the output cycle 151.

To minimize the amount of hardware duplication, four thermocoupleoutputs are connected to multiplexers 140 (shown in FIG. 3) controlledby GPIO pins on the DSPs 156 and 158. The output signal of themultiplexer 140 is appropriately conditioned before being fed as aninput to the ADC 172. There are two multiplexers 140 and twoconditioning circuits, one pair for each DSP 156 and 158. This allows ahardware failure for one DSP 156 or 158 to be caught by the other DSP158 or 156. Because of settle time associated with multiplexingthermocouples, only one thermocouple is measured for each output cycle151. In each output cycle 151 one of the temperature sensors is coupledto controller 128 through multiplexer 140. After the controller 128samples the temperature sensor's signal, the multiplexer 140 switchesits output to the next temperature sensor. The next temperature sensoris sampled during the next output cycle 151. Thus, the delay betweenthermocouple measurements in the illustrated embodiment is four outputcycles 151. This delay is accounted for in the compensator's poles andzeroes. In other suitable embodiments, however, the thermocouplemeasurements may be sampled more or less frequently within the scope ofthe present disclosure.

In addition to the four thermocouples, a “calibration” channel on themultiplexer 140 is used to measure a zero offset value. This offset isthen applied to the four actual thermocouple readings. To minimize thedelay to which the compensator is exposed, this measurement is takeninfrequently, and never while the system 100 is controlling thetemperature. Each DSP's software also samples a thermistor and uses thatmeasurement to calibrate for ambient temperature.

With reference to FIG. 7, to determine the desired power based ontemperature difference, the primary DSP 156 uses an infinite impulseresponse (IIR) filter implementation of pole-zero compensation. Thepoles and zeroes for this compensator have been determined using analogmodeling. When the generator is in diagnostic mode this portion of thecontrol is bypassed and the desired power for each electrode 112 issimply 0.5 W.

In the inner loop 180, the controller 128 determines a target outputvoltage for the power supply 126 that will achieve the desired powerdelivery to the electrode 112 (sometimes referred to herein as themaximum demand electrode) that has the highest desired power determinedin the outer loop 178. The energy that would be dissipated through themaximum demand electrode if it were coupled to the target output voltagefor the entire output cycle 151 is determined. The target output voltageis determined based on the desired energy dissipation for the maximumdemand electrode and an energy dissipation difference signal from aprevious output cycle 151. The energy dissipation difference signal isthe difference between the previous cycles desired energy dissipationthrough the maximum demand electrode and the actual energy dissipationthrough the maximum demand electrode. To determine the target outputvoltage for the buck regulator based on the difference, the primary DSP156 uses an IIR filter implementation of pole-zero compensation. Thepoles and zeroes for this compensator have been determined using analogmodeling. When the output cycle begins, the controller 128 causes thepower supply 126 (shown in FIG. 3) to operate to produce the targetoutput voltage.

As described above, the target output voltage for the power supply 126is determined in the inner loop 180 by the electrode 112 with thehighest desired power, and the output duty cycle for that electrode 112is the maximum possible duty cycle. Any electrode 112 with less demandis driven at a lower duty cycle, i.e., for less time in the output cycle151. As may be best seen in FIGS. 9 and 10, the maximum duty cycle is afunction of the number of enabled electrodes 112. Because of themeasurements that need to be taken at the beginning of the output cycle151, the output period 154 of the output cycle 151 is less than theentire output cycle 151. If only one electrode 112 is enabled, themaximum duty cycle is 100%. If more than one electrode 112 is enabled,the maximum duty cycle for an electrode 112 is reduced by the amount oftime that the electrode 112 will need to be disconnected whilemeasurements are being taken with other electrodes 112. Thus, themaximum duty cycle with four electrodes 112 enabled is 92.8% if themaximum demand electrode 112 is one of the electrodes 112 that isincluded in the combination measurement sub-period 152. In the exampleshown in FIG. 9, the maximum demand electrode 112 is not one of theelectrodes 112 included in the combination measurement sub-period 152.If, as shown in FIG. 9, the maximum demand electrode is not part of thecombination measurement sub-period 152, the maximum duty cycle is 90.4%.If only three electrodes 112 are enabled, the maximum duty cycle is95.2% for the electrodes 112 in the combination measurement sub-period152 and 92.8% for the electrode 112 not in the combination measurementsub-period 152.

The minimum duty cycle for an enabled electrode 112 is equal to the timethat it must be connected to the output to take measurements at thebeginning of the output cycle 151. If the required duty cycle for anelectrode 112 is less than or equal to the electrode's minimum dutycycle, the electrode 112 will be connected for its minimum duty cycleduring the measurement period 150 and will not be connected during theoutput period 154.

During an output cycle 151, the primary DSP 156 switches electrodes 112on or off by sending on or off pulses to a pulse transformer (notshown). As shown in FIG. 12, dead time between electrode 112 switchingis avoided by switching on the next electrode 112 before the current oneis switched off. This eliminates any electrical transient that mightotherwise result from this switching.

The length of the output cycle 151 is chosen to substantially minimizethe minimum duty cycle while not impacting responsiveness of the controlsystem. If the minimum duty cycle were too large, controller 128 wouldnot be able to deliver power low enough to keep electrodes 112 with verygood arterial contact from overshooting the temperature set-point. Ifthe output cycle 151 were too long, the control system would not be ableto react quickly enough to changing conditions.

As shown in FIG. 8, when the target output voltage is delivered to theelectrodes 112 within an artery 142, energy is dissipated in atherapeutic resistance 144 and a common return path resistance 146.Dissipation of energy through the therapeutic resistance 144 results inthe local temperature increases in the wall of the artery 142 desiredfor ablation. Energy dissipated through the common return pathresistance 146 does not increase the temperature in the wall of theartery and is thus considered a non-therapeutic dissipation.Accordingly, to accurately determine the energy dissipated in thetherapeutic resistance 144, the values of the therapeutic resistances144 and the common return path resistance 146 need to be determined. Thevalues for the resistances 144 and 146 will change based on location ofthe electrodes 112 relative to the return electrode 106, the quality ofthe contact between electrodes 112 and the walls of the artery 142, thetemperature of the walls of the artery 142, etc. Therefore, during themeasurement period 150 at the beginning of each output cycle 151,controller 128 acquires several measurements to allow it to determinethe common return path resistance 146 and the therapeutic resistances144.

The primary DSP 156 calculates impedance and power for each electrode112 based on a set of simultaneously sampled voltage-current pairs takenduring the measurement period 150 at the beginning of each output cycle151 using its ADC 172. Root mean square (RMS) voltage and RMS currentfor each electrode path, i.e., the path from the connected electrode(s)112 to the return electrode 106, are also calculated. RMS values areneeded because the output voltage that is being sampled is not directcurrent (DC). There is a single current sensor 149 for each DSP 156 and158 on the return path for all electrodes 112. Therefore, in order tomeasure the current flowing through a single electrode 112, thatelectrode 112 must be the only electrode 112 connected to the outputduring its measurement period.

Because the generator 102 operates with multiple electrodes 112simultaneously connected to the output, calculating the power dissipatedat a specific electrode 112 requires the common return path resistance146 to be determined and accounted for. To calculate this value,additional voltage and current measurements must be taken with at leasttwo electrodes 112 connected to the output. This measurement period isreferred to as the “combo” measurement period. If only one electrode 112is enabled, the common return path resistance 146 cannot be determined,but accurate computation of power delivered when only a single electrode112 is enabled does not require such a determination of the commonreturn path resistance 146.

During the measurement period 150 at the beginning of an output cycle151, voltage and current are measured with each enabled electrode 112connected to the target output voltage by itself while all otherelectrodes 112 are disconnected, and with two electrodes 112 connectedwhile all other electrodes 112 are disconnected (i.e., combo measurementperiod). As graphically shown in FIGS. 9-12, the measurement period 150of the output cycle 151 is divided into a number of equal lengthmeasurement sub-periods 152. In the illustrated embodiment, eachmeasurement sub-period 152 lasts for one hundred and twentymicroseconds. The number of measurement sub-periods 152 is determined bythe number of electrodes 112 to be enabled. For example, there is onemore measurement sub-period 152 than the number of electrodes 112 to beenabled. Accordingly, FIGS. 9 and 11 have five measurement sub-periods152 with four electrodes 112 enabled, and FIG. 10 has four measurementsub-periods 152 with three electrodes 112 enabled. During each of themeasurement sub-periods 152, except the last sub-period 152, a differentone of the electrodes 112 is coupled to the target output voltage.Voltage sensors 148 (FIG. 8) are sampled by the controller 128 tomeasure the voltage across the therapeutic resistance 144 of aparticular electrode 112 and the common return path resistance 146. Thepath from a particular electrode 112 through the common return electrode106 is sometimes referred to herein as a branch. A current sensor 149(shown in FIG. 8) is sampled by controller 128 to determine the currentthrough the branch to which the target output voltage is coupled. Duringthe last measurement sub-period 152, two electrodes 112 are coupled tothe target output voltage defining a combined branch. The voltagesensors 148 provide the voltage across the combined branch and thecurrent sensor 149 detects the common return path current. In otherembodiments, the combined branch may be measured during a sub-period 152other than the last sub-period 152.

For mitigation purposes, the secondary DSP 158 (shown in FIG. 4) alsocalculates impedance, average power, etc. for each electrode 112.Besides measuring voltage and current, it must also measure the dutycycle of the power output at each electrode 112, as controlled by theprimary DSP 156 (shown in FIG. 4), in order to calculate average power.The HRCAP module 176 (shown in FIG. 5) is used to make this measurementby capturing the times between “on” and “off” pulses generated by thePWM modules 174 on the primary DSP 156. Since there are eight signalsbeing captured and only four capture devices, four set-reset (SR)latches (not shown) are utilized. Each latch is set by an “on” pulse andreset by an “off” pulse, and the HRCAP modules 176 capture the high andlow times of the output signals from the latches.

Since the primary DSP 156 controls when the electrodes 112 are connectedto the output, a synchronization mechanism is required for the secondaryDSP 158 to take voltage and current measurements while electrodes 112are individually connected to the output. The HRCAP module 176 is usedfor this functionality as well, with interrupts being generated onrising edges. When the output of a latch connected to an HRCAP module176 transitions from low to high, it is an indication that theassociated electrode 112 has been connected to the output. When such aninterrupt occurs, if no interrupts have been generated on the otherHRCAP modules 176, the secondary DSP 158 can infer that the particularelectrode 112 is individually connected to the output and that it cankick off voltage and current sampling for that electrode.

An issue with using the HRCAP peripheral for measurement synchronizationis presented when an electrode 112 has been connected for the maximumduration during an output cycle 151 and is then about to be individuallyconnected for measurements at the start of the next output cycle 151. Inthis case, the associated SR latch will never be reset, and the HRCAPmodule 176 will not detect a rising edge to indicate that the electrode112 is now individually connected. To overcome this, the primary DSP 156generates a fifth PWM signal which is synchronized with the other PWMmodules 174 used for connecting and disconnecting electrodes 112. Thisfifth signal briefly disables the SR latches once per output cycle,ensuring that the HRCAP modules 176 on the secondary DSP 158 will alwaysdetect a rising edge for the first measurement sub-period 152 of anoutput cycle 151.

As shown in FIGS. 9-12, following completion of the measurement period150, the output period 154 begins and controller 128 couples the targetoutput voltage to all of the electrodes 112. The controller 128 uses thecurrent measured through each branch during the measurement period 150and the calculated therapeutic resistance 144 to determine the amount ofenergy dissipated through each electrode 112 during the measurementperiod 150 and to determine a remaining amount of energy to bedissipated through each electrode 112. The controller 128 generallykeeps each electrode 112 coupled to the target output voltage until theenergy dissipated through the electrode 112 during the output cycle 151reaches the desired energy dissipation determined based on the desiredpower calculation of the outer loop 178. The last electrode 112 coupledto the target output voltage is maintained coupled to the target outputvoltage until the end 155 of the output cycle 151. Thus, in theillustrated embodiment with all four electrodes 112 enabled, there arefour configurations during the output period 154. In order ofoccurrence, the configurations are: all four electrodes 112 coupled tothe target output voltage, three electrodes 112 coupled to the targetoutput voltage, two electrodes 112 coupled to the target output voltage,and one electrode 112 coupled to the target output voltage. At thebeginning of any configuration, the remaining energy to be dissipatedthrough an electrode 112 still coupled to the target output voltage isits desired energy dissipation less the energy dissipated during themeasurement period 150 and during any earlier configurations.

The amount of time each electrode 112 is switched on (i.e. electricallycoupled to the target output voltage, during the output period 154 ofthe output cycle 151) is determined by an iterative calculation based onthe desired energy dissipation through each electrode 112. Until allelectrodes 112 are dropped, the remaining energy to deliver iscalculated for each electrode 112 still switched on, (for the initialcalculation, energy delivered during the measurement period 150 is alsotaken into account). Based on the earlier measured output voltage, thecommon return path resistance 146, and the calculated resistances 144 ofthe remaining ‘on’ electrodes 112, updated branch currents arecalculated. Based on the updated branch currents and the remainingenergy to deliver to each electrode 112, the remaining on time for eachelectrode 112 is calculated. The on time for the electrode 112 withsmallest on time is recorded and that electrode is turned off for thenext iteration. Because at least one electrode 112 is switched on at alltimes and the output cycle is fixed at 5 milliseconds (ms), theelectrode 112. This electrode 112 with the largest on time (i.e., themaximum demand electrode) is left on for the entire duration of theoutput period 154. The difference between this electrode's actual ontime and desired on time is used to calculate the energy dissipationdifference that is used as feedback for the inner loop 180.

During the output period 154 following the measurement period 150 of theoutput cycle 151, thermistor and thermocouple measurements are taken.The temperature measurements cannot be taken while the ADC 172 is beingused for voltage and current measurements. Temperature measurements takeapproximately 50 microseconds (μs) each. Thermocouple measurementsrequire a 4 ms delay between switching the thermocouple multiplexer 140and taking the measurement in order for the signal to settle out. Forthis reason, only a single thermocouple measurement is taken during anoutput cycle 151. This means the shortest possible temperature loop 178(shown in FIG. 7) period is 20 ms, since each temperature is updatedonce every four cycles. After the thermocouple measurement is completed,the multiplexer 140 (shown in FIG. 3) is switched to the next channel.This results in approximately 5 ms between the multiplexer 140 beingswitched and the measurement being taken. Due to the thermocouple settletime, the calibration multiplexer channel is only measured while thetemperature control loop is inactive. Since any change should occurslowly and should not be severe, using calibration measurements that aretaken while the controls are inactive is sufficient. Similar to samplingof the voltages and currents, all temperature ADC sampling is handled bythe CLA 166. Once sampling is complete, the processor 136 averages andscales these samples to compute actual temperature values.

After temperature measurements have been taken and the thermocouplemultiplexer 140 has been updated, the remainder of the output cycle 151time is available for control loop computations. The outer loop 178corresponding to the electrode 112 with the currently updatedtemperature measurement is run, followed by the voltage loop 180. Thisyields an effective outer loop period of 20 ms and an inner loop periodof 5 ms. While the target output voltage of power supply 126 is setimmediately, the newly calculated electrode ‘on’ times are not put intoeffect until the next output cycle 151.

During the control loop calculations, the controller 128 uses themeasured voltages and currents for each branch, including the combinedbranch, to determine the value of the therapeutic resistances 144 andthe common return path resistance 146. For each electrode 112,controller 128 determines a branch resistance, which is the equivalentresistance of the combination of resistances 144 and 146, by dividingthe measured voltage by the measured current for that branch. Controller128 determines a combined branch resistance for the combined branch bydividing the measured voltage across the combined branch by the measuredcommon return path current for the combined branch. The common returnpath resistance 146 is determined by

RC=RX12−√{square root over ((RX1−RX12)*(RX2−RX12))}  (1)

where RC is the common return path resistance 146, RX12 is the combinedbranch resistance, RX1 is the branch resistance of the first branchincluded in the combined branch, and RX2 is the branch resistance of thesecond branch included in the combined branch. Because each therapeuticresistance 144 is in series with common return path resistance 146 inits respective branch, the therapeutic resistances associated with eachelectrode may be determined by subtracting the common return pathresistance 146 from its determined branch resistance.

As each electrode 112 reaches its desired energy dissipation and isdecoupled from the target output voltage, the current through theremaining electrodes 112 and the power dissipated through the associatedtherapeutic resistances 144 changes. Accordingly, controller 128calculates the power dissipation in the therapeutic resistances 144 andthe remaining energy to be dissipated in the therapeutic resistances 144for all of the configurations which will occur in the output cycle 151.Controller 128 uses the determined values of therapeutic resistances144, common return path resistance 146, and the target output voltage todetermine the current that will flow through each therapeutic resistance144 and the power that will be delivered to each therapeutic resistancein each configuration. For each configuration, the amount of energyremaining to be dissipated through each electrode 112 (desired energyless previously delivered energy) is calculated and divided by the powerthat will be dissipated in the associated therapeutic resistance 144during that configuration to determine how long the target outputvoltage would need to be coupled to each electrode 112 at the calculatedpower to achieve the desired energy dissipation. In each configuration,except the single electrode 112 configuration, the electrode 112 havingthe least amount of time remaining to reach its desired energydissipation determines when the configuration ends.

As the output period 154 progresses, the electrodes 112 are decoupledfrom the target output voltage at the times calculated as describedabove. For all but the last electrode 112, the desired energydissipation will have been reached. As described above, however, thelast electrode 112 coupled to the target output voltage remains coupledto the target output voltage until the end of the output cycle 151regardless of whether or not the desired energy for that electrode 112has been delivered. The difference between the desired energydissipation for the final electrode and the actual energy dissipationthrough that electrode is an energy dissipation difference that is usedas feedback for the determination of the target output voltage for thenext output cycle 151.

Example Timing Calculations

An example illustrating the timing calculations performed by system 100will now be described. In this example, system 100 includes threeenabled electrodes 112 referred to as E1, E2, and E3. The therapeuticresistances 144 associated with E1, E2, and E3 are identified asresistances R1, R2, and R3. RC is the common return path resistance 146.E1 and E2 were simultaneously coupled to the target output voltageduring the combination measurement sub-period 152 to form a combinationbranch. The measured branch currents for E1, E2, and E3 are identifiedby IX1, IX2, and IX3, respectively. The current through the combinationbranch during the combination measurement sub-period is IX12. Eachmeasurement sub-period (referred to in this example as ‘m’) is 2.4% ofthe output cycle 151 and the output cycle 151 is 0.005 seconds (referredto in this example as ‘Tmux’). For simplicity, measured and calculatedvalues are rounded in this example.

The previously determined desired amounts of power to be applied to E1,E2, and E3 are 3.7 watts (W), 4.3 W and 4.1 W, respectively. With anoutput cycle 151 of 0.005 seconds, the desired energy to be dissipatedthrough R1, R2, and R3 (via E1, E2, and E3 respectively) is 0.019 joules(J) for E1, 0.022 J for E2, and 0.021 J for E3.

During the measurement period 150, the target output voltage applied toE1, E2, and E3 was measured as fifty volts. The branch current IX1 wasmeasured as 0.333 ampere (A), the branch current IX2 was measured as0.25 A, the branch current IX3 was measured as 0.167 A, and thecombination branch current IX12 was measured as 0.41 A.

Branch resistances are calculated for each branch by dividing themeasured voltage by the measured branch resistance. Thus, for E1, fiftyvolts divided by 0.333 A gives a value of 150 ohms (Ω) for the branchresistance RX1. The branch resistances RX2 and RX3 are similarlycalculated to be 200Ω and 300Ω, respectively. Combination resistanceRX12 is calculated to be 121.875Ω. The common return path resistance 146is calculated using:

RC=RX12−√{square root over ((RX1−RX12)·(RX2−RX12))}=75Ω  (2)

For each branch, the therapeutic resistance is determined by subtractingRC from the branch resistance. Thus,

R1=RX1−RC=75Ω  (3)

R2=RX2−RC=125Ω  (4)

R3=RX3−RC=225Ω  (5)

After the individual resistive elements have been calculated, thecontroller 128 iteratively calculates the remaining on-times for eachelectrode 112. The measurement cycle has delivered a portion of thetarget energy to each load element and that energy must be subtractedfrom the target energy to compute remaining energy for each electrode112. The amount of energy remaining to be delivered to R3 through E3 is

JR3=J3−(m·Tmux·IX3² ·RX3)=0.0195J  (6)

where J3 is the previously determined amount of energy to be deliveredthrough E3. Because E1 and E2 were turned on individually and incombination, the calculation of the amount of energy delivered to R1 andR2 must take into account the energy delivered during both theindividual measurement and the combination measurement. Accordingly, theremaining energy to be delivered to R1 through E1 is

$\begin{matrix}{{{JR}\; 1} = {{J\; 1} - \left( {{m \cdot {Tmux} \cdot {IX}}\; {1^{2} \cdot {RX}}\; 1} \right) - {\quad{\left\lbrack {{m \cdot {Tmux} \cdot \left\lbrack \frac{v - \left( {{IX}\; {12 \cdot {RC}}} \right)}{R\; 1} \right\rbrack^{2} \cdot {RX}}\; 1} \right\rbrack = {0.01532J}}}}} & (7)\end{matrix}$

where J1 is the previously determined amount of energy to be deliveredthrough E1, and v is the measured voltage. The remaining energy to bedelivered to R1 through E2 is

$\begin{matrix}{{{JR}\; 2} = {{J\; 2} - \left( {{m \cdot {Tmux} \cdot {IX}}\; {2^{2} \cdot {RX}}\; 2} \right) - {\quad{\left\lbrack {{m \cdot {Tmux} \cdot \left\lbrack \frac{v - \left( {{IX}\; {12 \cdot {RC}}} \right)}{R\; 2} \right\rbrack^{2} \cdot {RX}}\; 2} \right\rbrack = {0.01943J}}}}} & (8)\end{matrix}$

where J2 is the previously determined amount of energy to be deliveredthrough E2.

At the beginning of the output period 154, all electrodes 112 are turnedon. The time needed to deliver the remaining energy to each electrode112 in this state (‘All-On’) is calculated for each electrode 112. Thelowest time calculated indicates which electrode 112 to turn off first.Initially, the branch currents need to be calculated for the All-Onstate. The voltage across RC is calculated as

$\begin{matrix}{{VRC} = {\frac{{RC}*v}{{RC} + \left\lbrack \frac{1}{\left( \frac{1}{R\; 1} \right) + \left( \frac{1}{R\; 2} \right) + \left( \frac{1}{R\; 3} \right)} \right\rbrack} = {32.955\mspace{14mu} V}}} & (9)\end{matrix}$

The current through each resistor is calculated by dividing the voltageacross the resistor (v-VRC) by the calculated resistance. This resultsin a current (I1) of 0.227 A through R1, a current (I2) of 0.136 Athrough R2, and a current (I3) of 0.076 A through R3. The remaining ontimes are calculated as

$\begin{matrix}{{{TR}\; 1} = {\frac{{JR}\; 1}{I\; {1 \cdot V}} = {1.348 \times 10^{- 3}s}}} & (10) \\{{{TR}\; 2} = {\frac{{JR}\; 2}{I\; {2 \cdot V}} = {2.85 \times 10^{- 3}s}}} & (11) \\{{{TR}\; 3} = {\frac{{JR}\; 3}{I\; {3 \cdot V}} = {5.148 \times 10^{- 3}s}}} & (12)\end{matrix}$

where TR1 is the remaining time that E1 must be connected in the currentstate (All-On) to deliver its targeted energy, TR2 is the remaining timethat E2 must be connected in the All-On state to deliver its targetedenergy, and TR3 is the remaining time that E2 must be connected in theAll-On state to deliver its targeted energy.E1 has the lowest time and will be the first to be switched off. Thiswill occur about 1.35 milliseconds after the measurement period 150ends, or 1.708 milliseconds into the output cycle, for a duty cycle ofabout 34.16%. In the controller 128, E1 is turned off and thecalculation sequence is repeated for the “Two-On” switch state.Initially, the remaining energy to be delivered is calculated bysubtracting the amount of energy delivered during the All-On state fromthe previously calculated remaining energy. This results in a new JR2 of0.01024 J and a new JR3 of 0.014391 The new voltage across RC iscalculated by

$\begin{matrix}{{VRC} = {\frac{RCv}{{RC} + \left\lbrack \frac{1}{\left( \frac{1}{R\; 2} \right) + \left( \frac{1}{R\; 3} \right)} \right\rbrack} = {24.138\mspace{14mu} V}}} & (13)\end{matrix}$

The current through each resistor is calculated by dividing the voltageacross the resistor (v-VRC) by the calculated resistance. This resultsin a current (I2) of 0.207 A through R2 and a current (I3) of 0.115 Athrough R3. The remaining on times are calculated as

$\begin{matrix}{{{TR}\; 2} = {\frac{{JR}\; 2}{12 \cdot V} = {9.89968 \times 10^{- 4}s}}} & (14) \\{{{TR}\; 3} = {\frac{{JR}\; 3}{13 \cdot V} = {2.50455 \times 10^{- 3}s}}} & (15)\end{matrix}$

E2 now has the lowest time calculated and will be the next output to beswitched off. This will occur 990 microseconds after the onset of the‘Two On’ switch state, or 2.698 milliseconds into the output cycle for aduty cycle of 53.96%.

The calculation sequence is repeated for the “One-On” state. Althoughthe last electrode 112 will remain on for the entire output period 154,the difference between the calculated time remaining and the timeremaining in the output period 154 is used to calculate the energydissipation difference term for use by the inner control loop 180 tochange the output voltage of the power supply 126. The remaining energyto be delivered is calculated by subtracting the amount of energydelivered during the Two-On state from the remaining energy calculatedin the last iteration. This results in a new JR3 of 0.00870447 J. Thenew voltage across RC is calculated by

$\begin{matrix}{{VRC} = {\frac{RCv}{{RC} = {R\; 3}} = {12.5\mspace{14mu} V}}} & (16)\end{matrix}$

and the current (I3) is calculated as 0.167 A. The remaining on time forE3 is:

$\begin{matrix}{{{TR}\; 3} = {\frac{{JR}\; 3}{13 \cdot v} = {1.04454 \times 10^{- 3}s}}} & (17)\end{matrix}$

E3 will hit its output energy target for this output cycle 151 byremaining on for 1.045 milliseconds after the onset of the ‘One-On’switch state, or 3.743 milliseconds into the output cycle, for a dutycycle of 74.86%. The output cycle, however is 5 milliseconds in length.The difference between the time when E3 should be turned off to meet itsenergy target and the end of the output cycle 151 is a difference valueof 1.257 milliseconds. This difference value indicates that surpluspower was delivered. The difference term is used as an input to theinner voltage loop 180 IIR filter to lower the output voltage of thepower supply 126.

Leaving the example and referring now to FIGS. 3-5, in addition tocontrol loop measurements, each DSP 156 and 158 must be able to updatewatchdog registers in the FPGA 160 before the FPGA 160 generates asystem error. The time before an error condition will be generated isapproximately 300 ms. While this is a relatively long time periodcompared to the control loop timings, it is not an insignificantconsideration given that communication with the FPGA 160 is accomplishedusing CAN communications.

Each DSP 156 or 158 also needs to communicate mitigation data to theother DSP 158 or 156. The data being exchanged must be agreed upon byboth DSPs 156 and 158. This data includes measurement data (e.g.,temperature) and state data (e.g., Ready or Diagnostic state). If thereis disagreement or a lack of communication for approximately 100 ms, atleast one of the DSPs 156 or 158 will generate a system error. A CANmessage will be sent to the FPGA 160 to discontinue output, and CANmessages will be sent to the UI processor 162 and to the other DSP 158or 156 indicating the error. Mitigation data is exchanged using theMcBSP peripheral every 10 ms.

To limit the potential consequences of critical component failure, thesecondary DSP 158 monitors the activities of the primary DSP 156. TheFPGA 160 is also designed to cut off all power if it does not receiveheartbeat packets within a certain timeframe from either DSP 156 or 158.

To mitigate the possibility of a program malfunction due to erroneousmemory or bus operation, the memory components attached to the systemare checked both at boot time and during normal operation. The flashmemory 138 is verified by computing a cyclic redundancy check (CRC) overits contents and comparing it to a previously stored value. RAM teststypically consist of three sections. One section tests the memory 138itself, and the other two test the data and address bus connections.Since the RAM in this implementation is on-chip, the bus tests are notrequired; if the buses are not operational, the chip will not function.The RAM test is executed once at system boot and continuouslythereafter, running in the lowest-priority task. The check itselfconsists of reading a memory location, storing its bitwise inverse atthe same location, re-reading the result, computing the bitwiseexclusive-OR with the original value and ensuring the result is bitwiseall ones before restoring the original value. This ensures that all thebits in the memory location can correctly store both ones and zeros.Interrupts must be disabled immediately prior to the first read of thevalue and re-enabled directly after restoring the original value.Furthermore, it is important to maintain the global state of theinterrupt enable/disable register, to avoid inadvertently re-enablinginterrupts after another routine has disabled them. Because theprocessor 136 has an 8-stage pipeline, a flush operation is requiredbefore interrupts are re-enabled after a memory test to preventpremature reads from returning corrupted values.

The correct function of the control loop regulating the operatingparameters of the catheter 104 (output power and temperature) depend onthree issues: the quality of the connection of the analog components tothe input of the ADCs 172, the correct operation of the ADCs 172themselves, and valid outputs to the control loop calculation. Thesecondary DSP 158, therefore, verifies both the connection of the analogcomponents and the ADC 172 values of the primary DSP 156 by measuringthe output current, voltage, and temperature of each electrode 112 viaindependent analog connections and its own ADCs 172. The sensor valuesread by the ADC 172 of the primary DSP 156 are compared to the valuesread by the ADC 172 of the secondary DSP 158, and vice versa.Periodically, both DSPs 156 and 158 exchange their conversion resultsand verify that the values agree.

Because the connection to each sensor (temperature, voltage, or current)is independent, but the sensor itself is shared between the two DSPs 156and 158, it is possible to detect a failing sensor by checking whetheror not the ADCs 172 of both DSPs 156 and 158 are reading values that areout of range. A poor connection between a DSP 156 or 158 and one of itssensors, or a faulty ADC 172 may be detected when differing values forthe same sensor are found on either DSP 172.

The output of the control loop calculation is verified to be withindefined limits by the secondary DSP 158 using calculated values from thecontrol loop of the primary DSP 156. The secondary DSP 158 will onlyissue an alert and abort ablation if the calculated values are out ofrange for longer than a certain period of time, to prevent frequentaborted ablation sessions due to brief transients that are of noconsequence.

Confirmation of the correct operation of all three processingcomponents, i.e., the FPGA 160 and the DSPs 156 and 158, of the system100 is established by the periodic transmission of RPC watchdog packetsfrom one node to the other two. A certain level of confidence isattained by the reception of periodic status messages by both DSPs 156and 158 from the FPGA 160, but this does not confirm the normaloperation of RPC packet reception by the FPGA 160. For this reason, bothDSPs 156 and 158 periodically transmit RPC watchdog packets to the FPGA160. A similar argument applies to the verification of operation of theprimary DSP 156 by the secondary DSP 158. Accordingly, RPC watchdogmessages from the secondary DSP 158 to the primary DSP 156 (and viceversa) guarantee prompt detection of problems in either component.

The frequency of the RF energy emitted is important to the ablationprocedure. For this reason the secondary DSP 158 decomposes the outputwaveform into its constituent frequencies via fast Fourier transform(FFT). This permits verification of the correct frequency setting aswell as the output waveform, to avoid the emission of unwanted higherfrequency sidebands.

In some embodiments, the determined common return path resistance 146(shown in FIG. 8) is also utilized by the controller 128 for additionalfeatures of the ablation system 100. As described above, the ablationsystem 100 (shown in FIG. 1) includes a single return electrode 106 thatis shared with all of the electrodes 112 resulting in a common returnpath for all of the electrodes 112. Energy dissipated in the commonreturn path resistance 146 does not increase temperatures close toelectrodes 112 and does not aid ablation. The larger the common returnpath resistance 146, the more energy is wasted in nontherapeuticdissipation. Accordingly, in some embodiments, controller 128 generatesa notification when the common return path resistance exceeds athreshold value, thereby alerting the operator that the common returnpath resistance 146 is too high. The operator may attempt to repositionthe return electrode 106 to reduce the common return path resistance146. Alternatively, or additionally, the operator may connect a secondreturn electrode (not shown) to generator 102 to reduce the commonreturn path resistance 146. The notification may be an audible or visualnotification, such as by using one or more of indicators 120. In someembodiments, the notification is a unitless visual notification. Forexample, the notification may be a green light if the common return pathresistance 146 is equal to or below the threshold value and a red lightif the common return path resistance 146 exceeds the threshold value.Additional threshold values and notifications may be added to increasethe resolution of the notification. In some embodiments, the controller128 is configured to disable the electrodes 112, i.e. decouple theelectrodes 112 from the target output voltage and prevent re-coupling tothe output voltage, when the common return path resistance 146 exceedsthe threshold. In other embodiments, the controller 128 is configured todisable the electrodes 112, when the common return path resistance 146exceeds a second threshold greater than the first threshold. Thus, thecontroller 128 may alert the operator when the common return pathresistance 146 exceeds a first threshold value and disable theelectrodes 112 if the common return path resistance 146 continues toincrease above a second threshold value.

Because the controller 128 determines the common return path resistance146 and the therapeutic resistances 144, the controller 128 is able toaccurately determine the power applied to the therapeutic resistances144. In some embodiments, by combining the accurate power measurementswith the temperature measurements for each electrode 112, controller 128determines the thermal gain for each electrode 112. The thermal gain ofan electrode 112 is a change in temperature produced by an amount ofpower. In the present disclosure, thermal gain is generally the ratio ofa change in temperature measured at an electrode 112 (degrees Celsius)to the amount of power applied to that electrode (in watts). The thermalgain of an electrode 112 may change depending on, for example, the sizeof the electrode 112, the material composition of the electrode 112, thesize of an artery in which the electrode 112 is located, the amount offluid surrounding the electrode 112, the thermal transfercharacteristics of the environment around the electrode, and the qualityof contact between the electrode and the wall of artery 142. For aparticular electrode 112 in a particular artery, a higher thermal gaingenerally indicates better apposition, or contact, with the wall of theartery than a lower thermal gain. Thus, controller 128 is configured toutilize the thermal gain of the electrodes 112 as an indication of thequality of contact between electrodes 112 and the artery wall. Thecontroller 128 generates a notification corresponding to the thermalgain, and thus the contact quality, for each electrode 112. Thenotification may be an audible or visual notification, such as by usingone or more of indicators 120. In some embodiments, the notification isa unitless visual notification. For example, the notification may be agreen light if the contact quality is good and a red light if thecontact quality is poor. Additional threshold values and notificationsmay be added to increase the resolution of the notification. Forexample, a numerical scale, e.g. integers one through ten, may bedisplayed to the operator with one end of the scale representing verygood contact quality and the opposite end indicating very poor contactquality. Numbers between the ends indicate graduation in quality betweenvery good and very poor. In some embodiments, the controller 128 isconfigured to disable the electrodes 112, i.e. decouple the electrodes112 from the target output voltage and prevent re-coupling to the outputvoltage, when the thermal gain is less than a threshold thermal gain.

In one particular embodiment, the controller 218 is configured to limita maximum power applied to the electrodes 112 based at least in part onthe determined thermal gain of each electrode 112 (sometimes referred toherein as adaptive power limiting). During operation, the maximum powerdelivered to each electrode is generally limited by a predeterminedpower limit. In one example, the power limit is eight watts. In otherembodiments, however, the power limit may be other than eight watts(higher or lower). The determined thermal gain of each electrode 112 iscompared to a predetermined thermal gain threshold that generallyindicates adequate contact quality. The thermal gain threshold is twentydegrees Celsius per watt (20° C./W). The exemplary thermal gainthreshold of twenty degrees Celsius per watt (20° C./W) was determinedbased on animal study data for nominal renal artery sizes of fivemillimeters to six millimeters. Other artery sizes and/or other factorsmay dictate use of a different thermal gain threshold (whether larger orsmaller). In another embodiment, the thermal gain threshold is aboutfifteen degrees Celsius per watt (15° C./W). In other embodiments, thethermal gain threshold may be any suitable threshold value determined toindicate a minimum arterial contact for an ablation procedure using aparticular ablation system. Each electrode 112 that has a thermal gainabove the threshold is controlled as described herein subject to thepredetermined power limit. Any electrodes for which the thermal gain isless than the thermal gain threshold are limited to a reduced powerlimit until the thermal gain for that electrode 112 reaches the thermalgain threshold.

The controller 218 determines the reduced power limit, for electrodes112 having a thermal gain less than the threshold thermal gain, as afunction of the amount by which the thermal gain of the electrode 112 isless than the thermal gain threshold. In a more particular embodiment,the reduced power limit is calculated by:

Plimit=MaxPlimit−ScalingFactor*(Threshold−ThermalGain)  (18)

where Plimit is the reduced power limit, MaxPlimit is the original(maximum) power limit, ScalingFactor is a scaling factor, Threshold isthe thermal gain threshold, and ThermalGain is the determined thermalgain. The scaling factor in one exemplary embodiment is 0.5. In such anembodiment, the controller 218 reduces the power limit by one half of awatt for each degree Celsius per watt that the determined thermal gainis below the thermal gain threshold. In another exemplary embodiment,the scaling factor may be 0.3. It is understood that in otherembodiments the scaling factor may be other than as set forth abovewithout departing from the scope of this disclosure.

The adaptive power limiting is not applied at the beginning of anablation procedure in order to permit the thermal gain values to reachequilibrium as the electrode temperature ramps up to the temperatureset-point. Rather, in one embodiment, the adaptive power limit featureis activated upon the occurrence of a suitable startup condition. Thestartup condition may be, for example, the first to occur of theelectrode temperature reaching a temperature threshold or the energydissipated through the electrode reaching an energy threshold. In oneexemplary embodiment, the temperature threshold is sixty-five degreesCelsius (65° C.) and the energy threshold is twelve joules (12 J).Alternatively, the temperature threshold and/or the energy threshold maybe any other suitable respective threshold. For each electrode 112,after at least one of the startup conditions has been met, the adaptivepower limit described above is applied to that electrode as applicable.

FIG. 24 is a flow diagram of one exemplary implementation of adaptivepower limiting in the ablation system 100. For this example, the maximumpower limit is eight watts, the thermal gain threshold is twenty degreesCelsius per watt (20° C./W), the scaling factor is 0.5, and the startupcondition is the first to occur of the electrode temperature reaching astartup temperature threshold or the energy dissipated through theelectrode reaching a startup energy threshold. An ablation procedurebegins at step 2400 and the power limit (PLimit) is set to eight watts.At 2402, the controller 218 determines whether or not the temperature(t) at an electrode 112 is greater than or equal to the startuptemperature threshold of sixty-five degrees Celsius (65° C.). If not,the controller determines at 2404 whether or not the energy dissipatedthrough the electrode 112 equals or exceeds the startup energy thresholdof twelve joules (12 J). If not, the controller 218 returns to 2402. Ifthe temperature exceeds the startup temperature threshold at 2402 or theenergy exceeds the startup energy threshold at 2404, the controllerproceeds to step 2406. As long as the determined thermal gain(ThermalGain) for the electrode 112 equals or exceeds the thresholdthermal gain of twenty degrees Celsius per watt (20° C./W), the powerlimit remains at the original setting (i.e., eight watts). If thedetermined thermal gain is less than the threshold thermal gain, thepower limit is reduced according to equation (18) at step 2408.Moreover, throughout the ablation procedure, the determined thermal gainmay be communicated to the operator of the ablation system 100 asdescribed above.

FIGS. 15A, 15B, and 15C and FIGS. 16A, 16B, and 16C graphicallyillustrate computer simulations of a portion of a single sixty secondlong ablation procedure using the adaptive power limiting describedherein. FIGS. 15A, 15B, and 15C are the graphical results of a simulatedablation procedure without use of the adaptive power limiting, whileFIGS. 16A, 16B, and 16C are the graphical results of a simulatedablation procedure using the adaptive power limiting. For both of theseexamples, the maximum power limit is eight watts. Beginning at aboutfive seconds into the ablation procedure, the temperature set-pointramps up from the ambient temperature (about forty degrees Celsius, 40°C.) to the ablation set-point of seventy degrees Celsius (70° C.).Contact between the electrode 112 and the artery wall decreases sharplyabout twenty seconds into the ablation procedure. These simulationsapproximate an arterial spasm decreasing contact of the electrode 112with the artery wall for about five seconds.

In particular, FIG. 15A presents the electrode temperature set-point1500 and the measured electrode temperature 1502. FIG. 15B shows thepower 1504 delivered to the electrode 112, and FIG. 15C is a graph ofthe thermal gain 1506 of the electrode 112. At approximately twentyseconds, the thermal gain 1506 drops rapidly from forty degrees Celsiusper watt (40° C./W) to five degrees Celsius per watt (5° C./W) and thetemperature 1502 decreases below the temperature set-point 1500. Thecontroller 218 attempts to increase the electrode temperature 1502 backto the set-point 1500 by supplying additional power 1504 to theelectrode 112. Because of the decreased arterial contact (andaccordingly decreased thermal gain 1506), the additional power is unableto increase the temperature 1502 back to the set-point 1500. The power1504 applied to the electrode 112 thus increases, without significantlyincreasing the temperature 1502, until the eight watt power limit isreached at about twenty-two seconds. When the thermal gain returns toforty degrees Celsius per watt (40° C./W) at about twenty-five seconds,the electrode temperature 1502 spikes up above the temperature set-pointdue to the large amount of power being applied to the electrode 112.

Turning now to the simulation illustrated by FIGS. 16A, 16B, and 16Cincluding the adaptive power limiting, FIG. 16A presents the temperatureset-point 1600 and the electrode temperature 1602. FIG. 16B shows thepower 1604 delivered to the electrode 112 and the power limit 1605. FIG.16C graphs the actual thermal gain 1606 of the electrode 112 and thedetermined thermal gain 1608. At approximately twenty seconds, theactual thermal gain 1606 drops rapidly from 40° C./W to 5° C./W and thetemperature 1602 decreases below the temperature set-point 1600. Thedetermined thermal gain 1608 decreases slower than the actual thermalgain 1606. While the determined thermal gain 1608 remains above athermal gain threshold of 20° C./W, the controller 218 attempts toincrease the electrode temperature 1602 back to the set-point 1600 bysupplying additional power 1604 to the electrode 112. Because of thedecreased arterial contact (and accordingly decreased thermal gain1606), the additional power is unable to increase the temperature 1602back to the set-point 1600. Once the determined thermal gain decreasesbelow 20° C./W, the controller 218 decreases the power limit 1605proportional to the difference between the determined thermal gain 1608and the threshold. The power 1604 applied to the electrode 112 islimited to the reduced power limit 1605, and the electrode temperature1602 continues to decrease. When the actual thermal gain returns to 40°C./W at about twenty-five seconds, the electrode temperature 1602 andthe determined thermal gain 1608 begin to increase gradually. As thedetermined thermal gain 1608 increases, the reduced power limit 1605increases. Once the determined thermal gain 1608 is above the 20° C./Wthreshold, the power limit 1605 has returned to the original eightwatts. As can be seen, the adaptive power limiting inhibits a spike inthe electrode temperature 1602 above the temperature set-point 1600, aswell as inhibiting application of significant power to the electrode 112when it is not in good contact with the artery wall.

FIG. 17 is a graphical presentation of the results of a bench-top testof a single sixty second ablation procedure using the ablation system100 including the adaptive power limiting described herein. FIG. 17presents the temperature set-point 1700, the electrode temperature 1702,and the electrode power 1704 versus time. The time scale is in onehundred millisecond (100 ms) increments. The maximum power limit in thistest is eight watts and the startup conditions are an electrodetemperature of sixty-five degrees Celsius (65° C.) or twelve joules (12J) of energy delivered. At about fifteen seconds into the procedure, thetemperature set-point begins to ramp up from ambient temperature toseventy degrees Celsius (70° C.). Four spasms, which cause the thermalgain to drop rapidly, are simulated at times t1, t2, t3, and t4. Duringthe first spasm, neither startup condition has been met and the adaptivepower limiting is not applied. The power 1704 increases rapidly untilthe power limit of eight watts is reached. The startup condition is metbefore the second, third and fourth simulated spasms. As can be seen,the reduced power limit is applied to prevent the power 1704 fromincreasing during the spasms and preventing the temperature 1702 fromspiking up following the spasms.

FIGS. 18-20 graphically present the results of three separate sixtysecond ablation procedure animal tests of the system 100 with theadaptive power limiting. Each figure represents the measurements of asingle electrode 112 during a different test. The time scale is in onehundred millisecond (100 ms) increments. The maximum power limit in thistest is eight watts and the startup conditions are an electrodetemperature of sixty-five degrees Celsius (65° C.) or twelve joules (12J) of energy delivered. The thermal gain threshold is twenty degreesCelsius per watt (20° C./W), and the electrode temperature set-point(not shown) begins to ramp up to seventy degrees Celsius (70° C.) atabout ten seconds into the procedure. All three figures present theelectrode temperature 1802, the electrode power 1804, the power limit1805, the thermal gain 1808 of the electrode 112, and the energy 1810dissipated through the electrode 112 as a function of time.

In FIG. 18, the startup condition is met and the adaptive power limit isactivated at about time t1. At time t1, the thermal gain 1808 isslightly less than the threshold thermal gain of 20° C./W and the powerlimit 1805 decreases, but the thermal gain 1808 returns above 20° C./Wbefore the power limit 1805 decreases enough to impact operation of theablation system 100. Throughout the ablation procedure shown in FIG. 18,the temperature 1802 remains fairly constant at the temperatureset-point of seventy degrees Celsius (70° C.) after it is ramped up tothat temperature.

In FIG. 19, the startup condition is met and the adaptive power limit isactivated at about time t1. At time t1, the thermal gain 1808 is lessthan the threshold thermal gain of 20° C./W and the power limit 1805decreases. The power 1804 delivered to the electrode 112 is limited tothe reduced power limit 1805. As the thermal gain 1808 varies after timet1, the power limit 1805 is correspondingly varied. After about time t2,the thermal gain, although varying, remains close to the threshold 20°C./W and the power limit 1805 remains close to the maximum power limitof eight watts. Throughout the ablation procedure shown in FIG. 19, thetemperature 1802 remains fairly constant at the temperature set-point ofseventy degrees Celsius (70° C.) after about time t2.

In FIG. 20, the startup condition is met and the adaptive power limit isactivated at about time t1. In this test, there is poor contact betweenthe electrode 112 and the artery wall. At time t1, the thermal gain 1808is less than the threshold thermal gain of 20° C./W and the power limit1805 decreases. The power 1804 delivered to the electrode 112 is limitedto the reduced power limit 1805. As the thermal gain 1808 varies aftertime t1, the power limit 1805 is correspondingly varied. After time t1,the thermal gain 1808 remains well below the threshold 20° C./W and thepower limit 1805 remains relatively low (around four watts) and limitsthe power applied to the electrode 112 for the remainder of the periodof the ablation procedure. In this test, the temperature 1802 is limitedby the low thermal gain 1808 and the power limit 1805, fluctuatingbetween about fifty-five and sixty-five degrees Celsius (55° C.−65° C.)after time t1.

With reference now to FIGS. 21-23, at the beginning of an ablationprocedure, the system 100 (and more specifically the controller 218)gradually increases the temperature at each electrode 112 until thetemperature reaches a temperature set-point. Ramping up the temperatureat the electrodes 112 allows lesion formation and desensitizes theartery at a lower temperature before the full ablation temperature isreached, which may reduce arterial spasms. FIG. 21 shows the temperature2100 of one electrode 112 for a sixty second ablation procedure. Thetemperature 2100 begins at about forty degrees Celsius (40° C., i.e.,around body temperature). At time t1, controller 218 selectively couplesthe electrode 112 to the generator output to increase the temperature2100 at a predetermined rate (in degrees Celsius per second) ofincrease. At time t2, the temperature 2100 has reached the temperatureset-point of seventy degrees Celsius (70° C.) and controller 218selectively couples the electrode 112 to the generator output tomaintain the temperature 2100 at the temperature set-point. Thepredetermined rate of increase applied to each electrode 112 may be thesame or different.

Moreover, the beginning of the temperature increase for each electrode112 may occur concurrently, or at different times. For example, FIG. 22shows the temperatures 2200, 2202, 2204, and 2206 of four electrodes E1,E2, E3, and E4 (collectively referred to as electrodes 112). Thebeginning of the temperature increase for each electrode 112 isstaggered by a delay period. Thus, the temperature 2202 on electrode E2begins to increase after the delay period 2208 following the beginningof the increase of the temperature 2204 on electrode E1. Similarly, thetemperature 2204 of electrode E3 begins to increase following the delayperiod 2210 and the temperature 2206 on electrode E4 begins its increaseafter the delay period 2212.

In some embodiments, the temperature of the electrodes 112 is increasedin more than one stage. FIG. 23 graphically presents an example of a twostage ramp up of the electrode temperature 2300 for an electrode 112. Itis also understood that more than two stages may be used. At time t1,the controller 218 begins to increase the temperature of the electrode112 at a first predetermined rate. When the temperature 2300 reaches afirst temperature set-point SP1, the controller 218 selectively couplesthe electrode 112 to the generator to maintain the temperature at theset-point SP1 for a dwell period (from time t2 to time t3). Followingthe dwell period, the controller 218 selectively couples the electrode112 to the generator to increase the temperature 2300 at a secondpredetermined rate to a second temperature set-point SP2. The controller218 then maintains the temperature 2300 at the second temperatureset-point SP2 for the remainder of the ablation procedure. The secondpredetermined rate is not the same as the first predetermined rate.Specifically, the second predetermined rate is slower than the firstpredetermined rate (i.e., fewer degrees Celsius per second). In anexemplary embodiment, the first rate is four degrees Celsius per second(4° C./second), the first temperature set-point is sixty five degreesCelsius (65° C.), the second rate is one degree Celsius per second (1°C./second), and the second temperature set-point is seventy degreesCelsius (70° C.). In other embodiments, any other suitable rate may beused for the first and second rates, including the same rate. Moreover,in some embodiments, the dwell period is omitted (or has a value of zeroseconds).

At various times during operation, the controller 128 samples the outputvoltage of the generator 102. The output voltage of the generator 102 isthe output of the RF output circuit 130 and is a generally sinusoidal,time invariant output signal having a known output frequency. In theillustrated embodiment, the output signal has a frequency of 480kilohertz (kHz). The illustrated controller 128 is capable of samplingthe output signal at a rate of 1.6 megsamples per second (MS/s).Accordingly, the controller 128 is able to acquire approximately threesamples of the output signal during one period of the output cycle. Toimprove the resolution of the sampling of the output voltage, thecontroller 128 samples the output voltage at different phases throughoutmany periods of the output signal and combines the multi-period samplesinto a representation of a single period of the output signal.

In general, the sampling method employed by the controller 128 involvessampling a time invariant output signal at a sampling rate that resultsin the samples of the output signal in any period of the output signalbeing acquired at different phases of the output signal than the samplesacquired during the immediately previous period. The shift in the phaseof the samples is fixed by the frequency of the output signal and thesample rate. Specifically, the sample shift in time is defined by:

sample shift=(n*sample period)−output period  (19)

where the sample period is the period of the sampling, the output periodis the period of the output signal, and n is the smallest integer valuethat results in a sample shift greater than zero. In order for thesampling method described herein to be used, the sample shift cannotequal zero. In other words the output period cannot be evenly divisibleby the sample period. If the sample shift equals zero, the output periodor the sample period can be adjusted to produce a nonzero sample shift.The amount of the phase shift may be found by:

$\begin{matrix}{{{phase}\mspace{14mu} {shift}} = {\frac{{sample}\mspace{14mu} {shift}}{{output}\mspace{14mu} {period}}*360{^\circ}}} & (20)\end{matrix}$

Because the frequency of the output signal and the sampling rate arefixed, the phase shift will advance the phase of the samples throughouta number of periods until the samples during a period of the outputsignal substantially align with the phases sampled in the first periodsampled. The number of samples (S) that are acquired before this occursis the smallest integer S that satisfies:

$\begin{matrix}{{{remainder}\mspace{14mu} {of}\mspace{14mu} \left( {S*\frac{{phase}\mspace{14mu} {shift}}{360{^\circ}}} \right)} = 0} & (21)\end{matrix}$

The number of output signal periods that are sampled before the samplephases realign with the first period sample phases, sometimes referredto herein as a superperiod, may be found by dividing the number ofsamples by the number of samples that may be acquired in one period ofthe output signal, i.e. n−1 samples. The resolution of the sampling is:

$\begin{matrix}{{resolution} = \frac{S}{360{^\circ}}} & (22)\end{matrix}$

The samples acquired during such a superperiod are combined as afunction of phase to produce a representation of a single period of theoutput signal.

Sampling Example

This method for sampling the output signal will be further illustratedwith reference to FIGS. 13 and 14. For this example, the output signalhas a period of 2.1 microseconds (μs), corresponding to a frequency of476.19048 kHz. The controller 128 samples the output signal at a rate of1.5385 megasamples per second (MS/s), giving a sample period of 0.65 μs.Using equation (19), four is the smallest integer value that results ina sample shift greater than zero. Because n equals four, the sampleshift is 0.5 μs. Plugging the sample shift into equation 3 results in aphase shift of about 85.7°. When S equals 42, equation (21) issatisfied. Thus, in this example, 42 samples will be acquired during asuperperiod, which will include 14 output periods, and combined torepresent one period of the output signal, with a resolution of 8.57°.FIG. 13 is a graphical representation of the output signal waveform1300. The diamonds on the output signal waveform 1300 indicate samples1302 of the output signal by controller 128. In FIG. 14, the samples1302 are graphed as a function of the output signal phase of the samples1302 producing a representation of a single period of the output signalwaveform 1300. Each sample is spaced from its neighbor samples by 8.57°(the sampling resolution). This provides a much more accuraterepresentation of a period of the output signal than would be providedby the three samples separated from each other by about 111° that areacquired during a single phase of the output signal.

Although described herein with reference to the output signal of theablation generator 102, this sampling technique may be used to sampleany suitable signal in any suitable apparatus. The signal to be sampledmust have a fixed and known frequency. The signal needs to besubstantially invariant, i.e., each period of the signal should be thesame as each other period of the signal. The more invariant the signal,the more accurate the combined representation will be. The signal doesnot necessarily need to be invariant over multiple superperiods.

Although certain embodiments of this disclosure have been describedabove with a certain degree of particularity, those skilled in the artcould make numerous alterations to the disclosed embodiments withoutdeparting from the spirit or scope of this disclosure. All directionalreferences (e.g., upper, lower, upward, downward, left, right, leftward,rightward, top, bottom, above, below, vertical, horizontal, clockwise,and counterclockwise) are only used for identification purposes to aidthe reader's understanding of the present disclosure, and do not createlimitations, particularly as to the position, orientation, or use of thedisclosure. Joinder references (e.g., attached, coupled, connected, andthe like) are to be construed broadly and may include intermediatemembers between a connection of elements and relative movement betweenelements. As such, joinder references do not necessarily infer that twoelements are directly connected and in fixed relation to each other. Itis intended that all matter contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative onlyand not limiting. Changes in detail or structure may be made withoutdeparting from the spirit of the disclosure as defined in the appendedclaims.

When introducing elements of the present disclosure or the variousversions, embodiment(s) or aspects thereof, the articles “a”, “an”,“the” and “said” are intended to mean that there are one or more of theelements. The terms “comprising”, “including” and “having” are intendedto be inclusive and mean that there may be additional elements otherthan the listed elements. The use of terms indicating a particularorientation (e.g., “top”, “bottom”, “side”, etc.) is for convenience ofdescription and does not require any particular orientation of the itemdescribed.

As various changes could be made in the above without departing from thescope of the disclosure, it is intended that all matter contained in theabove description and shown in the accompanying drawings shall beinterpreted as illustrative and not in a limiting sense.

1-20. (canceled)
 21. A method of beginning an ablation procedure using amulti-electrode ablation system, the method comprising: increasing atemperature at each electrode of a plurality of electrodes until thetemperature at each electrode reaches a first temperature set-point,wherein a respective rate of increase of the temperature at eachelectrode is limited to a respective predetermined first rate ofincrease at each electrode while the temperature at each electrode isincreasing to the first temperature set-point, and wherein therespective predetermined first rate of increase is not the same for eachelectrode of the plurality of electrodes; and following a respectivedwell period at each electrode after the temperature reaches the firsttemperature set-point, increasing the temperature at each electrode at arespective predetermined second rate of increase until the temperatureat each electrode reaches a second temperature set-point, wherein therespective dwell period is not the same for each electrode of theplurality of electrodes, and wherein the respective predetermined secondrate of increase is not the same for each of the plurality ofelectrodes.
 22. The method set forth in claim 21 wherein increasing atemperature at each electrode comprises increasing a temperature at eachelectrode beginning at substantially a same time for each electrode ofthe plurality of electrodes.
 23. The method set forth in claim 21wherein increasing a temperature at each electrode comprises increasinga temperature at each electrode beginning at a first time for at least afirst electrode and at a second time for at least a second electrode,wherein the second time is different from the first time.
 24. The methodset forth in claim 21 wherein increasing a temperature at each electrodecomprises increasing a temperature at each electrode beginning at adifferent time for each electrode of the plurality of electrodes. 25.The method set forth in claim 21 wherein a dwell period for at least oneelectrode has a value of zero seconds.
 26. The method set forth in claim21 wherein, for at least one electrode, the predetermined second rate ofincrease is less than the predetermined first rate of increase.
 27. Themethod set forth in claim 21 wherein the first temperature set-point isapproximately 65° C.
 28. The method set forth in claim 21 wherein thesecond temperature set-point is approximately 70° C.
 29. The method setforth in claim 21 wherein, for at least one electrode, the predeterminedfirst rate of increase is approximately 4° C. per second.
 30. The methodset forth in claim 21 wherein, for at least one electrode, thepredetermined second rate of increase is approximately 1° C. per second.31. A multi-electrode ablation system comprising: a power supplyconfigured to be coupled to a plurality of electrodes; and a controllercoupled to the power supply, the controller configured to: increase atemperature at each electrode of the plurality of electrodes until thetemperature at each electrode reaches a first temperature set-point,wherein a respective rate of increase of the temperature at eachelectrode is limited to a respective predetermined first rate ofincrease at each electrode while the temperature at each electrode isincreasing to the first temperature set-point, and wherein therespective predetermined first rate of increase is not the same for eachelectrode of the plurality of electrodes; and following a respectivedwell period at each electrode after the temperature reaches the firsttemperature set-point, increase the temperature at each electrode at arespective predetermined second rate of increase until the temperatureat each electrode reaches a second temperature set-point, wherein therespective dwell period is not the same for each electrode of theplurality of electrodes, and wherein the respective predetermined secondrate of increase is not the same for each of the plurality ofelectrodes.
 32. The system set forth in claim 31 wherein increasing atemperature at each electrode comprises increasing a temperature at eachelectrode beginning at substantially a same time for each electrode ofthe plurality of electrodes.
 33. The system set forth in claim 31wherein increasing a temperature at each electrode comprises increasinga temperature at each electrode beginning at a first time for at least afirst electrode and at a second time for at least a second electrode,wherein the second time is different from the first time.
 34. The systemset forth in claim 31 wherein increasing a temperature at each electrodecomprises increasing a temperature at each electrode beginning at adifferent time for each electrode of the plurality of electrodes. 35.The system set forth in claim 31 wherein a dwell period for at least oneelectrode has a value of zero seconds.
 36. The system set forth in claim31 wherein, for at least one electrode, the predetermined second rate ofincrease is less than the predetermined first rate of increase.
 37. Thesystem set forth in claim 31 wherein the first temperature set-point isapproximately 65° C.
 38. The system set forth in claim 31 wherein thesecond temperature set-point is approximately 70° C.
 39. The system setforth in claim 31 wherein, for at least one electrode, the predeterminedfirst rate of increase is approximately 4° C. per second.
 40. The systemset forth in claim 31 wherein, for at least one electrode, thepredetermined second rate of increase is approximately 1° C. per second.